At present, a transmission speed of 11 Mbps in IEEE 802.11b and a maximum transmission speed of 54 Mbps in IEEE 802.11a and IEEE 802.11g are defined for a wireless LAN (“WLAN”), and its supporting products are marketed. This allows a low-cost WLAN to become popular and increases the number of users to build a WLAN, thereby sharply increasing the popularity of a WLAN. Nowadays, a WLAN is used not only by companies for their services, but also by individual users for their home networks.
As a basic use method of a WLAN, there are two types of the infrastructure mode and ad hoc mode. With the former infrastructure mode, a WLAN connects by radio between an access point corresponding to a radio base station for portable telephones and a terminal (or station). With the latter ad hoc mode, a WLAN directly interconnects between a plurality of terminals without access points.
A WLAN uses an ISM (Industrial, Scientific and Medical) band, which needs not be licensed, and therefore transmission error frequently occurs. Even if narrowband transmission is performed, the WLAN is likely to be subject to the influence of noise and interference. Therefore, in the WLAN, it is necessary to take into account interference waves from, for example, a microwave oven or other radio transmitting devices. Also, if a terminal enters a dead point due to multipath fading in addition to noise, it is not possible to receive frames.
As a countermeasure for the above transmission error, IEEE 802.11 proposes a retransmission scheme with ACK's (ACKnowledgments). In the retransmission scheme with ACK's, the receiving side needs to return ACK's to all frames transmitted from the transmitting side. If all steps of returning ACK's to all of these frames success, operations are decided to have completed. Otherwise, the operations are decided to have failed. That is, if the transmitting source of data frames does not receive ACK's, the frames are decided to be lost. Regardless of whether the data frames are lost in the initial transmission or ACK's are lost upon transmission, if the transmitting source cannot receive the ACK's, the transmitting source retransmits the same data frames.
In a WLAN, while a predetermined station performs basic operations (described above) for one access point, communication is suppressed so that other stations do not interrupt the basic operations of the currently-used station, which prevents collisions.
The station side starting basic frame exchange, that is, the transmitting station, detects and corrects transmission error of data frames.
Transmission error has to be detected in the transmitting station. For example, the transmitting station decides error based on the fact that an ACK is not returned from the receiving station. Upon detecting error, the transmitting station retransmits a frame.
Also, the transmission station has a retransmission counter for counting the number of frame retransmissions, and increments the retransmission counter by one every time a frame is retransmitted. The retransmission counter is provided every frame or fragment. Also, the retransmission counter is classified into two types of the short frame retransmission counter and the long frame retransmission counter.
A WLAN secures reliability by retransmission. If a station that transmits a frame does not receive an ACK from an access point or the receiving station, the frame transmission is decided to have failed. When transmission fails, the retransmission counter is incremented by one. Further, when the number of retransmissions reaches a predetermined upper limit value, transmission of the frame is cancelled, which is reported to a higher layer of a protocol.
Patent Document 1: Japanese Patent Application Laid-Open Number 2004-88154